Conventionally, a plasma processing apparatus shown in FIG. 15 is used for performing plasma processing on a substrate, e.g., a semiconductor wafer (hereinafter, referred to as “wafer”) W. In this substrate processing apparatus 120, a processing gas is introduced into a processing space S within a chamber 121 and, at the same time, a high frequency power is supplied from high frequency power supplies 122 and 123 to a lower electrode 124. In the processing space S, an electric field is generated by the high frequency power supplied to the lower electrode 124. Molecules or atoms of the processing gas are excited by the electric field, thereby generating a plasma. At this time, the wafer W mounted on the lower electrode 124 is subjected to plasma processing by radicals or positive ions in the plasma.
However, as disclosed in Patent Document 1, when a high-density plasma is generated by supplying a high frequency power to the lower electrode 124 and the frequency of the high frequency power is set to be high, a high frequency current generated by the high frequency power tends to be focused near the center of the lower electrode 124. As a result, the density of the plasma generated in the processing space S becomes higher at a region facing the center of the wafer W (hereinafter, referred to as “central region”) than at a region facing the periphery circumference of the wafer (hereinafter, referred to as “peripheral region”).
FIG. 16 shows distribution of an electron density Ne in the region facing the wafer with respect to the frequencies of the high frequency power supplied to the lower electrode. Here, the distribution has been normalized based on the electron density of the central region.
In general, when a plasma of a processing gas is generated, electrons as well as positive ions and radicals are generated. Therefore, the distribution of the electron density substantially coincides with that of the plasma density. Further, as shown in FIG. 16, as the frequency of the supplied high frequency power is increased from about 27 MHz to 150 MHz, the electron density near the center of the wafer W becomes higher than the electron density of the peripheral region of the wafer W. Particularly, when the frequency of the supplied high frequency power is higher than about 60 MHz, the profile of the distribution of the electron density becomes distinctly curved upward with a vertex peaked at the vicinity of the center of the wafer W.
When the plasma is generated in the processing space S, a negative bias potential Vdc is generated on the surface of the wafer W mounted on the lower electrode 124. Further, the Vdc is determined by the amount of electrons reaching the lower electrode 124. Hence, the amount of electrons reaching the vicinity of center of the wafer W where the electron density is focused is increased, whereas the Vdc is decreased. In other words, the distribution of the electron density and the distribution of Vdc are inversely correlated with each other.
When the distribution of Vdc is not uniform, the current flows on the surface of the wafer W. At this time, as shown in FIG. 3 to be later described, when the charge amount of the current passing through a gate oxide film 153b of a semiconductor device formed on the surface of the wafer W exceeds a predetermined threshold value, the gate oxide film 153b is damaged or destroyed. The gate oxide film 153b is also damaged or destroyed if the amount of charges that are accumulated on the gate electrode 152 when the current flows exceeds a predetermined threshold value.
Therefore, in order to avoid the destruction of the gate oxide film 153b, the present inventors have suggested a plasma processing method for supplying a high frequency power to the lower electrode 124 in a pulse form and alternately repeating a plasma generation state and a plasma non-generation state in which a plasma is not generated in the processing space S at predetermined intervals (see e.g., Patent Document 2). In this plasma processing method, the continuous plasma generation time is set to be short enough that the amount of charges accumulated on the gate electrode 152 by the current does not exceed a threshold value and, then, a plasma non-generation state is followed.
Since the plasma generation state and the plasma non-generation state are alternately repeated at predetermined intervals, even if the surplus amount of charges are accumulated on the gate electrode 152 in any location on the wafer W during the plasma generation state, the surplus charges accumulated are spread out to be distributed throughout to a periphery thereof during the plasma non-generation state, thereby solving the problem of charge accumulation on the gate electrode 152. Accordingly, it is possible to prevent the increase of accumulated charges on the gate electrode 152, and also possible to prevent damages to the gate oxide film 153b. 